A layered structure shown in FIG. 10 is known for such layered film. A layered film 11 is configured by a polarizing plate 1 including a protective film 1b layered on both sides of a polarizer 1a via an adhesive layer, and a separator 2 layered on one side of the polarizing plate 1 via an adhesive layer 2a. When inspecting defects such as foreign substances, scratches, nicks and the like existing on the polarizing plate 1, light is irradiated from an appropriate light source onto the polarizing plate 1, a reflected light image or a transmitted light image is acquired via an imaging unit such as a line sensor or a two-dimensional TV camera, and defect detection is performed based on the acquired image data. When inspecting the polarizing plate 1, the image data is acquired with an inspection polarization filter interposed in a light path between the light source and the imaging unit. Normally, a polarizing axis (e.g., polarized absorption axis) of an inspection polarization filter is arranged in a state (cross Nicol) orthogonal to a polarizing axis (e.g., polarized absorption axis) of the polarizing plate 1 to be inspected. According to the cross Nicol arrangement, an all-black image is input from the imaging unit if defect is absent, and the corresponding portion does not become black if the defect is present. Therefore, defect can be detected by setting an appropriate threshold value.
However, in the case of a layered film in which the polarizing plate 1 includes the separator 2, the linear polarized light is converted to an elliptical polarized light since the separator 2 has a birefringence (retardation), and the polarizing plate 1 and the inspection polarization filter substantially will not be in a cross Nicol state. As a result, the defect inspection of the polarizing plate 1 in the layered film 11 cannot be accurately performed.
A polarizing plate inspection device disclosed in Patent Document 1 is known as a layered film defect detection device resolved with the above problem. The polarizing plate inspection device includes a light source and an inspection polarization filter for converting the light from the light source to a linear polarized light, wherein the linear polarized light is entered to a polarizing plate with protective film (corresponding to retardation layer), and defect detection is performed based on the transmitted light image. Furthermore, a retardation plate for compensating the birefringence of the light caused by the protective film is arranged on the light path transmitting through the polarizing plate with protective film from the light source. The phase change by the protective film is canceled and the birefringence of the light by the protective film is compensated by separately arranging the retardation plate. Moreover, a configuration example of arranging a variable polarization optical element in which the phase angle of light can be adjusted by voltage to compensate the birefringence by the protective film that slightly differs for every product is disclosed.
Patent Document 1: Japanese Laid-Open Patent Publication No. 2005-9919